Light emitting device with improved conversion layer

ABSTRACT

A light emitting device includes a substrate layer and a light conversion layer located on said substrate layer. The light conversion layer is a polycrystalline ceramic layer, and is positioned on the substrate layer by sintering.

This invention relates to the field of light emitting devices,especially LEDs. In today's LEDs, there is usually a substrate, on whichseveral further layers, amongst them one or more light emitting layersare provided. After finishing of the LED, a light conversion means islocated on the LED. This is done to convert some of the light, which isemitted by the light emitting layers, to a different wavelength, e.g.from blue to yellow, in order to achieve a white light for the LED. Inseveral applications, this light emitting means is a polycrystallineceramic material, such as polycrystalline YAG (Yttrium-Aluminum-Oxide)doped with Cerium.

However, such applications bear several drawbacks:

At first, the polycrystalline ceramic material is brought up on the LEDafter finishing of the layers, which is disadvantageous for someapplications. Second, the polycrystalline material needs a “glue” forsticking to the LED. Third, the distribution of the polycrystallinematerial for some applications cannot be controlled.

It is therefore an object of the present invention to provide alight-emitting device, which comprises a light conversion means withimproved properties.

Accordingly, a light-emitting device is provided comprising a substratelayer and a light conversion layer located on said substrate layer,whereby the light conversion layer is a polycrystalline ceramic layer,characterized in that the light conversion layer is positioned on thesubstrate layer by sintering.

The term “light conversion layer” means in particular, that the lightconversion layer comprises a material that is capable of absorbing thelight emitted by a light emitting layer present in the LED and emittinglight of a different wavelength, e.g. absorbing blue light from thelight emitting layer and emitting yellow or green light.

The term “polycrystalline ceramic layer” means in particular that thelayer is made out of a material, which has a median average diameter of≧1 μm and ≦100 μm, preferably of ≧10 μm and ≦50 μm.

“Sintering” in the sense of the present invention includes also bondingof the polycrystalline ceramic layer to the substrate layer by applyingan external pressure. Such techniques are known as sinter-bonding,diffusion bonding, hot uniaxial pressing (HUP) or hot recrystallizationpressing. “Sintering” in the sense of the present invention includes thebonding of monolithic ceramic layers with substrate layers as well asthe bonding of powder layers of the light conversion material to thesubstrate in one or more processing steps.

According to a preferred embodiment of the present invention, therefractive index n₂ of said light conversion layer is equal or largerthan the refractive index n₁ of the substrate layer. This bears theadvantage, that light that would otherwise be lost due to totalreflection at the substrate layer interface may enter the lightconversion layer and thus be used.

According to a preferred embodiment of the present invention, therefractive index n₂ of said light conversion layer and the refractiveindex n₁ of the substrate layer match: n₂−n₁≧0 and n₂−n₁≦1. By doing so,a good match between the substrate layer and the light conversion layercan be achieved. Preferably, the refractive index n₂ of said lightconversion layer and the refractive index n₁ of the substrate layermatch: n₂−n₁≧0 and n₂−n₁≦0,8, more preferably n₂−n₁≧0 and n₂−n₁≦0,4).

According to a preferred embodiment of the present invention, thecollateral edges of the light conversion layer are essentially matchedto the collateral edges of the substrate layer. “Essentially” in thesense of the present invention means in particular that the edges of thesubstrate layer and the light conversion layer fit in an extent asusually achieved in a cut and break process of a two layer sinteredceramic.

It is especially preferred that the light conversion layer does notextend itself sideways over the substrate layer.

By doing so, better performance behaviour of the LED which contains thesubstrate layer and the light conversion layer can be achieved.Especially with increasing LED chip size the LED efficiency and emissionuniformity increase.

According to a preferred embodiment of the present invention, the lightconversion layer has a thermal stability of ≦1000° C. and ≧200° C. for acontinuous heating time of ≧1 h and ≦100 h. This is especially preferredfor applications in which the LED tends to get quite hot during a longerrun-time. Preferably, the light conversion layer has a thermal stabilityof ≦1000° C. and ≧400° C., more preferred ≦1000° C. and ≧600° C. andmost preferred ≦1000° C. and ≧800° C. for a continuous heating time of≧1 h and ≦100 h.

According to a preferred embodiment of the present invention, the lightconversion layer is essentially free of organic material and/or siliconmaterial. This is especially preferred for applications, which employ alarger amount of heat to the LED. In these cases, such organic materialsand/or silicon materials tend to react thermally thus showingdiscoloration. “Organic materials” means especially organic polymers,“silicon material” means especially silicone polymers.

According to a preferred embodiment of the present invention, the ratioof the coefficient of expansion of the substrate layer towards thecoefficient of expansion of the light conversion layer is ≧1:2 and ≦2:1.Preferably, the ratio of the coefficient of expansion of the substratelayer towards the coefficient of expansion of the light conversion layeris ≧1:1.5 and ≦1.5:1, more preferred ≧1:1.25 and ≦1.25:1. By doing so,an essentially tension-free LED can be built. A preferred material forthe substrate layer and light conversion layer in this regard is Al₂O₃(sapphire) and YAG(Y₃Al₅O₁₂) respectively. The coefficient of thermalexpansion for sapphire is 8,6×10⁻⁶/K and for YAG 6,9×10⁻⁶/K. Thus theratio of the coefficient of expansion of the substrate layer towards thecoefficient of expansion of the light conversion layer is 1,24:1, whichis most preferred for the present invention.

According to a preferred embodiment of the present invention, the colortemperature of the light-emitting device has been adjusted by abrasionof the light conversion layer. In case the chromaticity of a LED is notexactly equal to a chromaticity of a blackbody radiator, the colortemperature with the nearest chromaticity match is chosen (also calledCorrelated Color Temperature).

This is one of the major advantages and preferred features of thepresent invention. It has been a predominant issue in the production ofLEDs that the color temperature of LEDs varies from one LED to anotherwithout the ability of changing the color temperature after fabricationof the LED. This problem appears especially in LEDs which use blue lightemitting materials and YAG:Ce as conversion materials. Since theAbsorption of the YAG:Ce material is rather narrow, only slight changesin the absorption bands and intensities of the blue light emittingmaterial will lead to dramatic changes in the color temperature.

In a preferred embodiment of the present invention, the colortemperature of the LED is therefore changed after fabrication of the LEDby abrasion of the light-emitting layer. Since the light-emitting layeris sintered, this abrasion can be achieved very easily, exactly andeffectively. It is in this context especially preferred that theabrasion is done by a laser process, although all other known abrasionprocesses used in the field may be employed as well.

According to a preferred embodiment of the present invention, the lightemitting device furthermore comprises a further light conversion means,whereby the further light conversion means emits light in a wavelengthdifferent from the light conversion layer. By doing so, a furtheremission band may be introduced in the LED. This embodiment isespecially of use in cases when the emitting material emits blue lightand the light conversion layer emits green or yellow light. In thesecases, a further light conversion means, which emits red light may beintroduced, thereby further improving the emitting behavior of the LED.This additional light conversion means may be provided on the LED by allknown techniques in the field. Of practical use are polycrystallinepowders of oxides, nitrides and mixtures thereof doped with Eu²⁺ or Eu³⁺ions. Additional ions for alteration of the absorption and/or emissionproperties may also be added.

According to a preferred embodiment of the present invention, thesubstrate layer is made of a material chosen from a group comprisingsapphire (Al₂O₃), zinc oxide (ZnO), Mg_(1−x)Zn_(x)Al₂O₄ with x being0≧x≦1 and mixtures thereof and/or the light conversion layer is made ofa material chosen from the group of garnet type materials A₃B₅O₁₂ (A=Y,Lu, Gd, La, Tb, Ba, Sr, Ca or mixtures thereof; B=Al, Ga, Si, Sc, Mg ormixtures thereof), whereby the light conversion layer may optionallycomprise doping materials facilitating the light absorption and emissionprocesses. Preferred doping materials are chosen from a group comprisingCerium, Pr, Eu, Sm, Nd, Tb, Ho, Er, Tm, Yb, Dy and mixtures thereof Thepreferred doping levels are ≦10 mol-% and ≧0.01 mol-%, more preferred ≦5mol-% and ≧0.025 mol-% and most preferred ≦3 mol-% and ≧0.05 mol-%.

Preferably the additional light conversion means—if an additional lightconversion means is used—comprises a material selected out of the groupcomprising AES:Eu (AE=Sr, Ca, Mg, Ba or mixtures thereof),AE₂Si_(5−x)Al_(x)N_(8−x)O_(x):Eu (AE=Sr, Ba, Ca or mixtures thereof) andmixtures thereof. The preferred doping levels are ≦10 mol-% and ≧0.01mol-%, more preferred ≦5 mol-% and ≧0.025 mol-% and most preferred ≦3mol-% and ≧0.05 mol-%.

According to a preferred embodiment of the present invention, thethickness of the light conversion layer is ≧20/[D %] nm and ≦60/[D %]nm, whereby D % means the molar ratio of the doping material. It hasbeen shown in practice, that this thickness is most suitable for theconversion and thus for the obtainment of a desired color temperature.

A light emitting device according to the present invention may be of usein a broad variety of systems and/or applications, amongst them one ormore of the following:

-   -   household application systems    -   shop lighting systems,    -   home lighting systems,    -   accent lighting systems,    -   spot lighting systems,    -   theater lighting systems,    -   fiber-optics application systems,    -   projection systems,    -   self-lit display systems,    -   pixelated display systems,    -   segmented display systems,    -   warning sign systems,    -   medical lighting application systems,    -   indicator sign systems, and    -   decorative lighting systems.    -   portable systems

automotive applications.

The present invention also includes a method of preparing a LEDcomprising the steps of:

(a) providing a substrate layer area

(b) sintering a light conversion layer area on one side of the substratelayer area

(c) providing a light emitting layer area on the side of the substratelayer opposite to the side, where the light conversion layer area islocated

(d) dicing the obtained structure to obtain LEDs each comprising asubstrate layer and a light conversion layer

(e) measuring the color temperature of a LED

(f) adjusting the color temperature of the LED by abrasion of the lightconversion layer

whereby the steps (e) and (f) may be repeated ad libitum and for one ormore of the LEDs.

This method bears in particular the advantages as set out above, amongstthem:

-   -   LEDs, in which the collateral sides of the substrate layer and        the light conversion layer are matched and    -   LEDs, which color temperature has been set in an easy and        effective way.

It should be noted, that it is a prominent feature of a method accordingto the present invention, that the light conversion layer is providedprior to the light emitting layer. This especially for the reason thatby doing so, also materials can be of use for the light emitting layer,which are thermally instable during the sintering of the lightconversion material.

The present invention also includes a program for executing the steps(d) and/or (e) and/or (f) of the method as described above. Among othersa program is provided to apply a defined current to the LED, measuringthe light emission, calculating the color temperature and controlling amachine for abrasion of the light conversion layer. Measurement of theemitted light and calculation of the color temperature can be performedintegrally. In another embodiment emitted light is projected on adiffusing plate and measured with a camera (CCD) device to determine thearea resolved color temperature. A connected LASER abrasion system isthen controlled for position resolved abrasion of the light conversionlayer to achieve a predetermined color temperature profile on thediffusion plate.

The aforementioned components, as well as the claimed components and thecomponents to be used in accordance with the invention in the describedembodiments, are not subject to any special exceptions with respect totheir size, shape, material selection and technical concept such thatthe selection criteria known in the pertinent field can be appliedwithout limitations.

Additional details, characteristics and advantages of the object of theinvention are disclosed in the subclaims, the figures and the followingdescription of the respective figures and examples—which in an exemplaryfashion—show several preferred embodiments of a light emitting deviceand a method of producing such a light emitting device according to theinvention.

FIG. 1 shows a schematic cross-sectional partial view of a substratelayer and a sintered light conversion layer according to the invention

FIG. 2 shows a schematic perspective view employing the method ofproducing the LEDs according to the invention prior to step (d)

FIG. 3 is a graph, in which the change of the color properties of a LEDaccording to the invention by abrasion is shown

FIG. 4 is a graph employing the connection between the color temperatureand the thickness of the light conversion layer for a series of fourexemplarily LEDs

FIG. 5 is a graph showing the emission spectrum of a LED according toExample 2.

FIG. 1 shows a schematic cross-sectional partial view of a substratelayer 10 and a sintered light conversion layer 20 according to theinvention. As can be seen from this schematic view, the collateral edges10 a, 10 bof the substrate layer and the collateral edges 20 a, 20 b ofthe light conversion layer are matched towards each other, so that thelight conversion layer 20 does not exceed sideward above the substratelayer 10. The dotted lines in FIG. 1 show, where further layers or parts(30) of the LED are preferably provided, such as a light-emitting layer.

FIG. 2 shows a schematic perspective view employing the method ofproducing the LEDs according to the invention prior to step (d). As canbe seen, there is a substrate layer area 50 provided, on which a lightconversion layer area 60 has been located. Further parts of layers ofthe LED are usually also present on the side of the substrate layer area50 which is opposite to the side, where the light conversion layer area60 has been provided (indicated by the dotted lines). Altogether, astructure 70 is formed. When the LED is finished, the structure 70 iscut into pieces as indicated by the dot-and-line formed lines. By doingso, LEDs can be obtained, in which the collateral side walls of thesubstrate layer and light conversion layer are matched and the lightconversion layer does not exceed sideways over the substrate layer.

In FIG. 2, the LEDs, which are produced by cutting or slicing of thestructure 70 are—when seen in top view—somewhat rectangular in shape.However, it goes without saying, that by the method as presented by thepresent invention, LEDs of any shape may be produced.

FIG. 3 is a graph, in which the change of the color properties of a LEDaccording to the invention by abrasion is shown. FIG. 4 is a graphemploying the connection between the color temperature and the thicknessof the light conversion layer for a series of four exemplarily LEDs.After the manufacture of the LEDs, the LEDs will in most cases havedifferent color temperatures. By employing the present invention it ispossible to adjust this color temperature by abrasion of the lightconversion layer.

In an example, an LED might have the color temperature as represented bycolor point A in FIG. 3. By abrasion of the light conversion layer, itis possible to shift the color temperature and properties as indicatedby the dots towards color point B. However, it is for most applicationsdesired to reach the line 100 (the so-called Black Body line). In thiscase, the abrasion will only be done until color point C is reached,which is on the Black Body-line.

However, it should be noted, that for some applications it might beuseful to adjust the color properties to a different color point, e.g.color point D. This is useful for cases in which an additional lightconversion means, e.g. a red light emitting light conversion means isused. When the light conversion means is also applied on the LED, thecolor properties will be shifted to color point E, which is on the BlackBody line.

In these cases the following procedure has shown to be most effective:

-   -   Production of the LED    -   Adjusting the color temperature to a desired point, e.g. point D    -   Providing the light conversion means, e.g. by covering the LED        with a polycrystalline powder,    -   thus obtaining a shift of the color properties of the LED to        color point E, which is on the Black Body line

It goes without saying that this procedure is of best use, when thelight conversion means, in this case a red light conversion means, isless sensitive to shifts in the emitting spectra of the LED than thelight conversion layer, since it is in most cases not possible to adjustthe light conversion means after application. However, for lightconversion means, which emit red light, this is usually the case and thecolor point D, which needs to be adjusted can easily be calculated andso the endpoint E can be reached with a good accuracy.

As can be seen from FIG. 4, the thickness of the light conversion layerdramatically affects the color temperature of the LED. In FIG. 4, fourexemplary LEDs were analyzed with emitting material, which emits lightin the wavelengths of 440 nm, 456 nm, 462 nm and 468 nm, respectively.The LEDs each had a light conversion layer comprising YAG:Ce with adoping level of 2 molar-%. FIG. 4 shows, that simply by reducing thethickness of the light conversion layer by abrasion, a broad range ofcolor temperatures can be reached quite easily.

By choosing the optimum thickness for the light conversion layer, thedoping level of the doping material needs to be taken into account. Ithas been shown, that the thickness of the light conversion layer is≧20/[D %] nm and ≦60/[D %] nm, whereby D % means the molar ratio of thedoping material. For the exemplarily LEDs shown in FIG. 4, the optimumthickness of the light conversion layer is therefore ≧10 nm and ≦30 nmfor 2 mol-% doping with Ce.

The process of sintering according to the invention is—in a merelyexemplarily fashion—furthermore illustrated by the following examples:

EXAMPLE 1

-   a) Manufacturing of a Y₃Al₅O₁₂:Ce (YAG:Ce) Polycrystalline Ceramic

40 g Y₂O₃, 32 g Al₂O₃ and 3.44 g CeO₂ are mixed with 1.5 kg Al₂O₃milling balls (>99.9 wt % purity, 2 mm diameter) and ball milled withisopropanol for 12 hrs. The milled slurry is dried using a spray dryer.The granulated powder is then calcined in alumina boats at 1300° C. for2 hrs under a carbon monoxide atmosphere.

The obtained YAG:Ce powder is des-agglomerated with a planet ball mill(agate balls, under ethanol) and then the slurry is slip-cast in aplaster mold. After controlled drying the green bodies (100 mm diameter,2 mm thickness) are sintered at 1700° C. for 2 hrs on graphite platesunder a carbon monoxide atmosphere.

One surface of the YAG:Ce ceramic is then finished by polishing. Thedensity of the obtained YAG:Ce ceramic is >98% of the theoreticaldensity.

-   b) Diffusion Bonding of the YAG:Ce Ceramic and a Sapphire Waver

A sapphire—YAG:Ce composite is diffusion bonded by means of hot uniaxialpressing (HUP). For this purpose a sapphire waver and the polishedYAG:Ce ceramic were positioned between tungsten sheets (0.5 mmthickness) and put into a graphite die. To increase the production rateseveral tungsten/sapphire/YAG stacks can be stacked one above eachother.

After evacuation of the HUP apparatus it is first heated up to 1700° C.within 4 hrs without applying an external pressure. Then the pressure isincreased to 5000 PSI and kept constant for 2 hours. Then the system iscooled down to 1300° C. within 2 hours under constant pressure followedby cooling to room temperature within 6 hours.

-   c) Post Annealing of the Sapphire/YAG:Ce Composites

After polishing of the YAG:Ce surface sapphire/YAG:Ce composite isannealed at 1250° C. in air for 2 hours (heating and cooling ramps: 100K/hr).

EXAMPLE 2

GaN MQW light emitting layers were deposited on the sapphire surface ofa 2″ sapphire/YAG:Ce substrate/light conversion waver as described inExample 1. With electrical probe contacts individual LEDs on thesubstrate were operated consecutively at 40 mA current. Spectralemission was recorded, while an excimer LASER was used to abrase theYAG:Ce light conversion layer in the area of the emitting LED until aCIE 1931 color point was obtained sitting on a line given by:y=(0.415-0.303)/(9.485-9.697)*(x-0.485)+0.415. (This line is defined bythe CIE 1931 chromaticity of the red emitting CaS:Eu phosphor and aselected white point close to the Black Body line at x=0.485, y=0.415for a color temperature of 2400 K.) Then the substrate with the lightemitting layer and the adjusted light conversion layer was diced andindividual LEDs were packaged into a reflecting cup and connected toexternal electrical contact pads. Then a slurry of 1 percent by weightof poly-vinyl-alcohol (PVA) and 0.01 percent by weight of ammoniumdichromate (ADC) in water with 10 percent by weight of CaS:Eu phosphorwas dispersed on the die. After drying and exposure to UV light a robustphosphor layer was formed.

FIG. 5 shows the emission spectra of one LED according to Example 2. Itcan be seen that a white LED with a low color temperature and a goodcolor rendering can be achieved.

1. A light emitting device comprising: a substrate layer of Al₂O₃; and afirst light conversion layer located on said substrate layer and havinga doping material, said substrate layer being of a different material ascompared to said first light conversion layer: wherein said first lightconversion layer is doped at doping levels of ≦3 mol-% and ≧0.05 mol-%:and wherein said first light conversion layer has a thickness of ≧10 nmand ≦30 nm; wherein said first light conversion layer and said substratelayer have collateral edges which are matched; wherein the first lightconversion layer comprises a polycrystalline ceramic layer of YAG dopedwith Ce, wherein a refractive index n₂ of said first light conversionlayer is equal or larger than a refractive index n₁ of said substratelayer and wherein n₂−n₁≧0 and n₂−n₁≦1 wherein the light conversion layeris positioned on the substrate layer by sintering, and wherein a colortemperature of the LED is adjustable by abrasion of the light conversionlayer.
 2. The light emitting device according to claim 1, furthercomprising a second light conversion layer affixed to said first lightconversion layer, said second light conversion layer selected out of thegroup of AES:Eu wherein AE is selected from the group Sr, Ca, Mg, Ba andmixtures thereof, AE₂Si_(5−x)Al_(x)N_(8−x)O_(x):Eu wherein AE isselected from the group of Sr, Ba, Ca and mixtures thereof, and mixturesthereof.
 3. The light emitting device according to claim 1, wherein thefirst light conversion layer has a thermal stability of ≦1000° C. and≧200° C. for a continuous heating time of ≧1 h and ≦100 h.
 4. The lightemitting device according to claim 1, wherein the light conversion layeris essentially free of organic material and/or silicon material.
 5. Thelight emitting device according to claim 2, wherein said second lightconversion layer emits light in a wavelength different from the firstlight conversion layer.
 6. The light emitting device according to claim1, wherein the substrate layer is made of a material chosen from a groupcomprising sapphire (Al₂O₃), zinc oxide (ZnO), Mg_(1−x)Zn_(x)Al₂O₄ withx being 0≧x≦1 and mixtures thereof; and/or the light conversion layer ismade of a material chosen from a group comprising garnet type materialsA₃B₅O₁₂, where A=Y, Lu, Gd, La, Tb, Ba, Sr, Ca or mixtures thereof, andB =Al, Ga, Si, Sc, Mg or mixtures thereof; wherein the light conversionlayer comprises a doping material facilitating light absorption andemission processes, the doping material being chosen from a groupcomprising Cerium, Pr, Eu, Sm, Nd, Tb, Ho, Er, Tm, Yb, Dy and mixturesthereof.
 7. The light emitting device according to claim 1, wherein athickness of the first light conversion layer is ≧20/[D % ] nm and≦60/[D % ] nm, where D % is a molar ratio of the doping material.